However researchers at Cardiff University have found that offering this discount had no effect.

But charging an additional 25p per cup increased the usage of re-useable cups by 3.4%

And if you displayed environmentally friendly messages and provided some free re-useable cups the use of such cups increased by 12%.

This is a good demonstration of behavioural economics. People are more sensitive to losses than gains when making decisions. Charging 25p more is seen as a loss and people are more sensitive to that than saving the same amount.

“If we really want to change a customer’s behaviour then a charge on a disposable cup is more likely to be effective” said the author of the research report.

Many people might find it difficult to remember to take a re-useable cup with them. I have one in my car but hardly ever use even when visiting Costa at my local Tesco (partly because I don’t think Cappuccino travels well).

Last year Starbucks temporarily doubled the discount to 50p but it only increased the use of re-useable cups by 0.2% up to 1.2%.

After the success of charging people for plastic bags in shops it’s worth considering a similar move with coffee cups.